In this study, we demonstrate an efficient optical-to-thermal energy-conversion (self-heating) ability of room temperature (RT)-grown gallium nitride (GaN) thin films on direct amorphous glass substrates. Radio frequency (rf) magnetron sputtering was employed to experimentally achieve the RT crystallinity of these films directly on glass substrates. Finite-difference time-domain (FDTD) simulations are implemented to elucidate their optothermal response. Our experimental approach precludes the need for conventional prerequisites such as pre- or postannealing, substrate heating, or utilizing buffer (nucleation) nanolayers during the fabrication of GaN thin films on glass substrates. Notably, thin films with an excellent hexagonal wurtzite polymorphic phase comprising mainly (101), (002), and (100) planes is witnessed. Comprehensive characterizations, including structural, morphological, elemental, and surficial analyses, were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) with selected area electron diffraction (SAED), and atomic force microscopy (AFM), correspondingly. These analyses confirm the exceptional homogeneity, morphology, and structural integrity of the RT-grown GaN films on the glass. Furthermore, this study also examined the influence of rf-power and gas flow on the film growth rate, thickness, and overall quality. Thickness (tGaN) dependent optical response, power absorption (Pabs), and volumetric power dissipation (Pabs-density) in the GaN films were analyzed across the ultraviolet-visible (UV-visible) spectrum using FDTD simulations. Our findings offer a potentially transformative approach for the cost-effective, large-scale production of GaN substrates that are crucial for power and optoelectronic applications. This work paves the way for futuristic advancements by facilitating further investigation into critical challenges such as strain-induced crystallographic orientation issues, phase stability, and mitigation of lattice mismatch and dislocation defects in ultrathin GaN films. Furthermore, our research plays a vital role in unlocking the full potential of GaN films in energy harvesting applications.